Add a WindowsLocatorFactory to parse Windows %PATH% env var.

Windows not only uses a different separator (';' instead of ':'), but it
also looks in the current working directory first before checking other
directories.

This by no means handles windows support (#5), but it does close #4 as
the path separator change is handled here.
Add class docblocks for the locator factories.
Add a platform factory that wraps the Posix/Windows factories.

This factory allows the user to not worry about whether they are on a
POSIX system or a Windows system (or some other systems in the future).
It determines which factory to use by a Windows-specific constant
PHP_WINDOWS_VERSION_BUILD that should only be defined on Windows
systems.

I'm unsure about how this will work on Cygwin systems that make some
things operate in a more POSIXy way.
Use the PlatformLocatorFactory in documentation.

More people are likely to want to use the factory that works on all
platforms.
Document the PosixLocatorFactory and WindowsLocatorFactory.